The present invention relates to base paper for offset printing, particularly to newsprint paper for offset printing, which has a high whiteness degree, excellent water absorption resistance and ink acceptability by providing a coated layer of an inorganic surface preparation agent mainly comprising colloidal silica or silica sol, which has fewer adhesion problems such as surface tack, curling caused by adhesion and has low paper powder accumulation at offset printing and strike through after offset printing, and which contains low chemical oxygen demand (COD) load in wastewater when recycled is low.
In recent years, printing technologies have made great progress in conversion to offset printing, color printing, high-speed mass printing and automation of printing. With the advance of technologies, regarding printing paper as well, improvement of its various physical properties is demanded from workability and printability aspects.
Particularly, offset paper (newsprint paper, rolls of newsprint) is generally a type of paper mainly comprising mechanical pulp and deinked pulp (xe2x80x9cDIPxe2x80x9d). Even though the offset paper is classified into medium- or low-grade paper, it has more rigorous quality requirements for it than for general printing paper, because the specified number of copies needs to be printed without fail within the specified time in the specified time slot. Consequently, newsprint paper is special paper which can be uniquely classified in paper classification. Since lightness in weight, high DIP composition, etc. are currently demanded for newsprint paper, various improvements on newsprint paper need to be made while overcoming the minus factors. Thus, it is more difficult to improve newprint paper in comparison to general high-quality printing paper. For that reason, diverting technologies used for high-quality printing paper directly to newsprint paper is difficult. Conversely, it is relatively easy to divert technologies used for newsprint paper to general printing paper.
In newspaper printing, conversion from letterpress to offset printing is rapidly progressing along by the introduction of more computer systems in light of the demands for speeding up and automating printing, adding more color supplements and various kinds of printing, etc.
This recent diffusion of offset printing demands newsprint paper to possess qualities different from those of newsprint paper for letterpress. Quality requirements for offset newsprint paper include, for example, 1) low surface tack, i.e., good peeling property, 2) low paper powder accumulation on a blanket, 3) no web paper breaks caused by dampening solution during offset printing while maintaining adequate water-absorption resistance, 4) adequate print ink setting property, 5) high opacity and no strike-through, and 6) adequate friction coefficients. Among these quality requirements, particularly 1) lowering of surface tack, 2) improving of water absorption resistance, 3) improving of print ink acceptability, 4) high opacity and no strike-through, etc. are important issues that need to be addressed.
General printing paper possesses a low content of mechanical pulp and a high content of hardwood bleached kraft pulp (LBKP), while newsprint paper possesses a high content of mechanical pulp and DIP. Therefore, newsprint paper has more miniaturized fibers, and a problem with paper powder is more likely to occur. Additionally, when a mechanical pulp content is high, cohesion between miniaturized fibers is weak and a paper surface state is coarse. Consequently, paper powder falls off from the paper surface and tends to increase the paper powder accumulation on the blanket during the printing.
In 1989, 96% of newsprint paper had a basis weight of 46 g/m2 and in 1993, 80% of newsprint paper had a basis weight of 43 g/m2. Since newsprint paper is becoming lighter and lighter, problems such as lowered opacity and lowered paper strength, etc. occur. To compensate for these problems, a compounding ratio of an inorganic and an organic filler or pigment needs to be increased. However, by increasing a compounding ratio of a filler and a pigment, it facilitates the filler or the pigment to desorb, coupled with thinner and lighter paper. In the case of offset printing using a dampening solution, a tendency in desorption further increases because the dampening solution weakens cohesion between pulp fibers. These desorption tendencies become more serious problems as newsprint paper is getting lighter and lighter. For example, it becomes increasingly difficult to improve the newsprint paper of basis weight below 40 g/m2 than to improve the newsprint paper of basis weight above 43 g/m2.
Furthermore, a higher compounding ratio of DIP brings about an increase in DIP-derived miniaturized fibers and increased filler and pigment quantities and causes problems such as increased paper powder and lowered paper strength, coupled with a lighter weight of the paper. Additionally, DIP manufactured from magazine recycled waste paper contains tacky substances comprising a hot-melt adhesive, an acrylic adhesive and a vinyl acetate adhesive, which are used as a glue for the back of a magazine. If these tacky substances exist in a large quantity on a paper surface, the tacky substances adhere to a canvas, a belt, a roll surface, etc. and contact the paper when the paper passes through a paper manufacturing machine and/or a printing press causing such problems as web paper breaks and holes (adhesion troubles). Particularly, in the case of an offset web press type that controls paper running by bringing a steel belt into contact with a roll surface, this tendency is prominent. As a paper surface adheres to the steel belt, web paper breaks or holes (curling caused by adhesion) occur.
Improving paper surface strength was considered as a measure for addressing these newsprint paper problems. Known measures of improving the surface strength of newsprint paper are divided roughly into those by non-coating and those by coating.
Measures by non-coating include methods by changing a compounding ratio of raw materials, changing papermaking conditions, increasing a quantity of paper strength fortifier, etc. However, with these methods, it is difficult to respond to the rigorous quality requirements of newsprint paper.
Measures by coating are methods for coating (externally applying) a starch, a modified starch (oxidized starch, starch derivatives, etc.) or surface preparation agents such as polyvinyl alcohol on the surface of base paper for newsprint. These measures have become effective means for improving the surface strength.
From an economic standpoint, on-machine coating of a surface preparation agent on the newsprint paper is a commonly employed method. A gate roll coater, a method for forming and transferring a film with which high-speed coating is possible, is used. Characteristics of this gate roll coater method are summarized, for example, in Proceedings of the Japan Technical Association of the Pulp and Paper Industry Vol. 43, No. 4 (1989), p.36, the Paper Pulp Technical Times Vol. 36, No.12 (1993), p.20 among others. The method using the gate roll coater makes it possible to efficiently lay a coating solution on the paper surface and is useful for improving the paper surface as compared with the two-roll size press method used for general printing paper. More specifically, in the two-roll size press method the coating solution deeply penetrates into the base paper as the base paper passes through a pond of the coating solution whereas, in the gate roll coater method, penetration of the coating solution into the base paper is considerably controlled, because the coating solution forms a film beforehand and the film formed is transferred onto the paper surface. For this reason, a coating layer is formed evenly on the surface of the base paper with the gate roll coater method.
To control curling caused by adhesion, which is mentioned above, there are such means as removal or micro scattering of a tacky substance from raw materials, change in a compounding ratio of raw materials, change in papermaking conditions and coating of a surface preparation agent. Currently in the art, centrifugal processing and cleaner processing or mechanical scattering cannot achieve complete removal or scattering. Changing a compounding ratio of raw materials and papermaking conditions did not resolve the curling caused by adhesion.
As measures for improving the paper surface strength, a method for (externally) coating a surface preparation agent comprising a water-soluble high polymer such as a starch, a modified starch, polyvinyl alcohol, etc. has been known. To control the curling caused by adhesion by this method, it is necessary to coat a large quantity of the surface preparation agent. However, during the offset printing or newsprint paper manufacture, by using a large quantity of the surface preparation agent, the stickiness caused by the surface preparation agent increases under the influence of a dampening solution (a phenomenon called xe2x80x9cNepparixe2x80x9d). Due to the Neppari, this method was entirely unserviceable.
In the case of an offset press used for full-color printing, which is making headway in newspaper printing, a problem with Neppari is even prominent because a paper surface is dampened with water four times. Additionally, poor wet ink acceptability (i.e., a phenomenon of poor acceptability of black ink after cyan-magenta-yellow color printing) can easily occur.
When coating a conventional surface preparation agent using a film transfer method such as a gate roll coater, it is difficult to improve poor ink acceptability and surface tack (Neppari) at offset printing. Additionally, when tacky substances were mixed in a paper surface from the papermaking process using the raw materials and an internal size, it was extremely difficult to prevent problems such as curling caused by adhesion.
To add to the above, eluting into wastewater during the recycling process, a surface preparation agent comprising a water-soluble high polymer increases a chemical oxygen demand (xe2x80x9cCODxe2x80x9d) load and a biological oxygen demand (xe2x80x9cBODxe2x80x9d) load providing a nutrition source for microorganisms, and facilitating generation of slime. There is a possibility that slime, filler and hydrophobic constituents become a complex particle, further presenting new problems.
Aspects of the present invention provide printing paper having characteristics such as good peeling property, low paper powder accumulation on the blanket, no paper breaks caused by dampening solution during offset printing with water-absorption resistance properly maintained, adequate print ink setting property, high opacity and no strike-through, and adequate friction coefficients for the purposes of responding to current printing circumstances, i.e., increased use of offset printing, color printing, and high-speed mass printing, and automation of printing.
More specifically, the present invention can provide offset paper with sufficient surface strength and improved surface tack and curling caused by adhesion and excellent ink acceptability although the offset paper contains DIP that includes tacky substances. The present invention further can provide the offset paper with less COD load in wastewater in the recycling process.
The inventors of the present invention have found that curling caused by adhesion, ink acceptability and Neppari can be improved by coating an inorganic surface preparation agent mainly comprising silica sol or colloidal silica on base paper for printing and have solved the above-mentioned issues. Additionally, the inventors have also found that opacity can be improved simultaneously by adding an inorganic pigment in addition to silica sol or colloidal silica.
As examples of using colloidal alumina or silica sol or colloidal silica as a surface preparation agent, in the Japanese Patent Laid-open No.1992-12879 journal, a case of printing by coating water dispersions, in which a surface-active agent is added by below 5 parts by weight to 100 parts by weight of colloidal alumina on various matters to be printed including synthetic resin films, was disclosed. In the Japanese Patent Laid-open No.1992-327297 journal, an anti-slip agent, in which colloidal alumina and colloidal silica are mixed along with whisker, was disclosed. In the Japanese Patent Laid-open No.1994-48022 journal, for improving fixation of a toner for carbonless copy paper, as an inorganic colloid coating composition, colloidal alumina and silica sol are described. In the Japanese Patent Laid-open No.1998-131091 journal, newsprint paper for electrocoagulation printing on which a coating layer mainly comprising silica sol or colloidal silica, and binder is provided, is described. Furthermore, to prevent slip by increasing a friction coefficient of corrugated paperboard, use of colloidal silica is mentioned by Inoue and others (M. Inoue, N. Gurumagul, and P. Aroca, Tappi Journal, 72(12), 81-85, 1990). Similarly, C. H. Fletcher discusses that colloidal silica is used as a material for increasing friction in a report titled xe2x80x9cSlip Prevention Processing Using Colloidal Silicaxe2x80x9d (Refer to C. H. Fletcher, Tappi Journal, 1973, 56(8), 81-85).
There is more literature regarding improved yields of a filler and microscopic fibers during the papermaking by adding colloidal silica to a paper material. However, in the above-mentioned literatures in the public domain, there are no descriptions found regarding improving of curling caused by adhesion and surface tack (Neppari) and improved ink acceptability at offset printing by coating an inorganic surface preparation agent mainly comprising silica sol on printing paper, particularly newsprint paper.
As mentioned above, recent trends in newsprint paper (lighter weight, or promotion of a high compounding ratio of DIP) have become great minus factors in terms of surface strength. To address the trends, increasing a coating quantity of a surface preparation agent has become necessary. If using a large quantity of a surface preparation agent such as starches, modified starches or polyvinyl alcohol, which have been commonly used, effects on improving surface strength were manifested. However, there were problems caused by surface tack (a phenomenon called xe2x80x9cNepparixe2x80x9d) in the newsprint paper manufacture process or when printing, because such surface preparation agents present surface tack when dampened by water. These Neppari problems were notably serious when coating the surface preparation agent was done by a coat transfer method such as a gate roller coater as compared with a case where a two-roll size press was used. Furthermore, in the case of four-color printing, because of dampening the same printing surface of newsprint paper with dampening solution four times, a water membrane was easily generated on the blanket and a paper surface, and a phenomenon of poor acceptability of hydrophobic ink was observed.
Additionally, if newsprint paper does not have water absorption resistance, dampening solution easily penetrates into the newsprint paper during offset printing. Due to this, the strength of a portion dampened decreases and this is more likely to result in web paper breaks (referred to as xe2x80x9cpaper breaks caused by dampening solutionxe2x80x9d hereinafter) by tension at printing. This has become an important issue. Furthermore, when reading printed newspaper, xe2x80x9cstrike throughxe2x80x9d implying that characters and color photograph images printed on the backside of the newsprint paper can be seen through is very troublesome. For coating by a film transfer method such as a gate roll coater, improving surface strength of newsprint paper (controlling a quantity of paper powder), improving water absorption resistance, lowering surface tack (Neppari), improving ink acceptability and strike through measures have been extremely important issues.
Regarding these issues, for example, in the Japanese Patent Laid-open No.1994-57688 journal and in the Japanese Patent Laid-open No.1994-192995 journal, surface tack eliminators for improving surface tack by adding it to a surface preparation agent were disclosed. In the Japanese Patent Laid-open No.1994-57688 journal, a surface tack eliminator comprising an organic fluoro compound was disclosed, and in the Japanese Patent Laid-open No.1994-192995 journal, a surface tack eliminator, in which substituted succinic acid and/or substituted succinic acid derivatives are active constituents, was disclosed. Since using these surface tack eliminators reduces Neppari, it is effective for increasing a coating quantity of a surface preparation agent. Regarding using these surface tack eliminators, however, it was found that there were problems such as: 1), effervescence at the time of coating is significant because a coating material comprises two constituents, a surface preparation agent and a surface tack eliminator, 2) cost increase, and 3) decreased black ink acceptability at color printing.
In the Japanese Patent Laid-open No.1993-59689 journal and the Japanese Patent Laid-open No.1993-296693 journal, sizing agents for paper comprising polyvinyl alcohol and a polyether compound were reported. Particularly in the former, Japanese Patent Laid-open No.1993-59689, a case that by coating a composition comprising a block copolymer of ethylene oxide and propylene oxide, and polyvinyl alcohol on base paper for newsprint, surface strength improves and newsprint paper with low tack at offset printing can be obtained, was disclosed. Although this composition was able to improve surface tack to a certain degree as compared with starches or polyvinyl alcohol coated independently, satisfactory ink acceptability and surface tack were not be able to be obtained if lighter paper and a higher DIP compounding ratio would further advance.
Consequently, the inventors of the present invention proposed use of colloidal silica as a surface preparation agent for newsprint paper in the Japanese Patent Laid-open No.1999-233238. If colloidal silica was used independently, excellent surface strength and surface tack (Neppari) were provided. However, due to low opacity, there was xe2x80x9cstrike throughxe2x80x9d problem implying that characters and color photograph images printed on the backside of newsprint paper can be seen through. Additionally, because printing ink acceptability is slightly inferior, there was a problem in finished print. For needs for providing offset paper having excellent high whiteness, surface tack (Neppari), water absorption resistance and ink acceptability as well as few strike through occurrence after offset printing, these problems were solved by providing a coating layer containing a surface preparation agent comprising two constituents, colloidal silica and inorganic salt.